It is known that changes in vehicle loading cause changes in braking capability. For instance, when a vehicle is in the fully loaded condition, the rear wheels will have nearly the same braking capability as the front wheels. However, when the same vehicle is in the unladen condition, the rear wheels may have substantially less braking capability than the front wheels. Thus, the potential for premature rear wheel lock may be much greater when stopping the unladen vehicle than when stopping the fully loaded vehicle. This is particularly apparent in truck-type vehicles and subcompact passenger vehicles.
Vehicle hydraulic brake systems have included various types of pressure control valves which reduce the potential for premature rear wheel lock during brake applications. Conventional pressure control valves limit the rear wheel brake pressure relative to the front wheel brake pressure in response to a predetermined level of master cylinder pressure. One such type of pressure control valve is shown in U.S. Pat. No. 3,423,936. In the unladen vehicle condition, this type of valve is advantageous since it prevents premature rear wheel lock yet allows the brake system to produce the total brake torque that is needed for stopping the unladen vehicle in a predetermined distance. However, this type of valve is not entirely desirable in the fully loaded vehicle condition when there is a substantial difference between the fully loaded weight and the unladen weight of a vehicle such as in a truck. The principle reason that this type of valve may be undesirable in the fully loaded condition is that the brake system may not be able to produce the total brake torque that is needed for stopping the fully loaded vehicle in the same distance as the unladen vehicle.
In an attempt to overcome this problem, a similar type of pressure control valve has included a "blend-back" type bypass feature which steadily increases the rear brake pressure subsequent to a predetermined period when the rear brake pressure is limited. The brake system is thus able to provide the total brake torque that is needed for stopping the fully loaded vehicle in the same distance as the unladen vehicle. This type of valve has a disadvantage in that the wear rate of the front brakes is increased when the vehicle is fully loaded. Another disadvantage of this type of valve is that the potential for front wheel lock is increased when stopping the fully loaded vehicle on wet or icy surfaces.
These undesirable features and disadvantages of the prior art pressure control valves are more evident on truck-type vehicles and subcompact passenger vehicles than on intermediate and full-size passenger vehicles.